16.54: Phenakite

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Phenakite
Chemical composition	Beryllium sillicate Be₂SiO₄
Crystal system	Trigonal
Habit	Tabular to prismatic
Fracture	Conchoidal (brittle)
Cleavage	Indistinct (one direction)
Hardness	7.5 - 8
Optic nature	uniaxial +
Refractive index	1.651 - 1.670
Birefringence	0.016
Dispersion	Low, 0.015
Luster	Vitreous
Specific gravity	2.92 - 2.97
Pleochroism	Seen in strongly coloured stones
Fluorescence	Sometimes pale greenish / blue or pale rose

Figure \(\PageIndex{1}\): A faceted colorless gem-quality phenakite.
Photo courtesy of Chaman Golecha, Gem Testing Laboratory, Jaipur

Crystallography

Trigonal system.
Habit: Tabular, prismatic, rhombohedral, acicular.

Diagnostics

Diaphaneity

Transparent.

Color

Colorless; also yellow, pink, brown, pinkish red due to surface stains.

Refractometer

Phenakite is uniaxial with a positive optic sign.
Refractive index range: n_ω = 1.651-1.654 and n_ε = 1.663-1.670.
The full birefringence is 0.016.

Polariscope

Not many gemstones are uniaxial + (zircon, quartz, phenakite and some idocrase). Finding the optic sign with the use of a retardation filter should narrow down the possibilities.
Quartz will almost always show a typical bull's-eye pattern (or Airy spirals).

Spectrum

Nothing diagnostic.

Fluorescence

Phenakite sometimes shows pale greenish / blue or pale rose fluorescence in UV light.
Under X-ray it fluoresces blue.

Pleochroism

Dichroism in observed in strongly colored stones.

Magnification

May have many needle inclusions; 2-phase; fingerprints with phase inclusions.

Simulants

Name	Key separation
Spodumene	Magnification features, UV, biaxial figure, cleavage and S.G
Sillimanite	S.G., R.I., birefringence, biaxial Figure
Rock Crystal	Lower S.G. (floats 2.88), bulls eye optic figure, R.I.
Topaz	More heft (sinks 3.33), biaxial figure, R.I.
Zircon	S.G., R.I., birefringence

Occurrence

Geological occurrence: Commonly in pegmatites, mica schists, in granite drusses.

Geographical locations: Tokajawa river, Ural Mountains with emerald and chrysoberyl; Miask district, Rissia; large colourless crystals at San Miguel di Praciavaba, Minas Gerais; Tanzania; Maine, New Hampshire & Colarado, USA; Switzerland; Austria; Alsace, France; Namibia; Norway and Sri Lanka (large stone).